The vast reserves of coal in this country and throughout the world, have prompted and continue to prompt considerable interest and investigation into economical processes for the transformation of coal solids into liquid products that can be upgraded to provide synthetic petroleum fractions. The present invention provides such a process and is believed to represent a major break through in coal liquification technology, largely due to the fact that the process is designed to be carried out under normal atmospheric pressure.
The ability to convert coal to liquid hydrocarbon products is principally dependent upon the presence of relatively weak chemical bonds in the very large coal molecules which when thermally or catalytically cracked yield carbon free radicals. If hydrogen is available to react with the free radicals, desirable lower molecular weight hydrocarbons are produced.
Most, if not all, prior art coal liquification processes involve high-pressure systems. Even in those processes nominally referred to as "low pressure", the required pressure in one or more phases of the operation is anywhere from 20 to 90 atmospheres (300-1350 p.s.i.). Moreover, the prior art coal liquification (hydrogenation) processes generally all require relatively high temperatures (460.degree. to 750.degree. C.). The expense and engineering difficulties of operating at high temperatures and hydrogen pressures of 300-4000 p.s.i. and higher, result in such high production costs that the products are not commercially competitive with those produced from crude oil.
While conventional catalyst systems are generally employed to selectively accelerate the desired reactions essential to coal liquification processes, rapid deactivation of the ctalysts often result from the high hetroatom content of the coal, and the presence of polynuclear aromatic structures. Moreover, hydrogen sulfide, formed from the sulfur in coal, and ammonia formed from nitrogen, and oxygen deactivate acid cracking catalysts.
Accordingly, although it is known that hydrogenation and cracking of coal will produce a synthetic petroleum liquid product, difficulties such as those enumerated above have prevented realization of commercially efficacious processes for the conversion of coal to useful liquid fuels.